Ball valves are used in a wide number of process control system applications to control some parameter of a process fluid (this may be a liquid, gas, slurry, etc.). While the process control system may use a control valve to ultimately control the pressure, level, pH or other desired parameter of a fluid, the control valve basically controls the rate of fluid flow.
Typically, a ball valve may include a fluid inlet and a fluid outlet separated by a ball element which, by rotating about a fixed axis and abutting a seal assembly, controls the amount of fluid flow therethrough. During operation, the process control system, or an operator controlling the control valve manually, rotates the ball element against, or away from a surface of the seal assembly, thereby exposing a flow passage, to provide a desired fluid flow through the inlet and outlet and, therefore, the ball valve.
Ball valve components, including the ball element and the seal assembly, are typically constructed of metal; this stands especially true when used in high pressure and/or high temperature applications. However, the ball element and seal assembly suffer wear due to the repeated extensive engagement of the ball element and seal assembly during opening and closing of the valve. The problems resulting from the wear include, but are not limited to, diminished life span of the valve components, increased frictional forces between the ball element and the seal assembly, and undesirable leakage between the ball element and the seal assembly. Similarly, because the frictional forces tend to increase as the components become more worn, the dynamic performance and control characteristics within the valve are worsened, resulting in inefficiencies and inaccuracies in the valve.
In the past attempts have been made to incorporate a biased main seal into the seal assembly to correct the above mentioned problems. Some heavy duty designs have incorporated to a Teflon® radial seal to enhance sealing performance under high-temperature operations. Ball valves having Teflon® radial seals are able to achieve Class IV shutoffs up to operating temperatures of approximately 550 deg F. Above 550 deg. F, a graphite piston ring is currently used because Teflon® deteriorates above approximately 550 deg. F. Graphite piston rings, while able to withstand higher temperatures, are only capable of achieving Class III shutoffs. As a result, ball valves having a Class IV shutoff capability above approximately 550 deg. F are not currently available.
Therefore, there remains a need for a high-temperature ball valve having a Class IV shutoff capability for systems operating above approximately 550 deg. F.